How is called when someone carries his birth city as surname?

Sometimes history figures are called by its birth city as surname. Example: Milon of Croton, Thales of Miletus.

How is this called in English? Is this an Epithet?

Solution 1:

It is not epithet since that is an adjective added to a person's name or a phrase used instead of it, usually to criticize or praise them:

So epithet could be used for Richard the lionhearted (Richard Cœur de Lion)

However epithet is one form of by-name and another form is nobiliary particle

If he was also known as Richard lord of Aquitaine amongst many other titles.

and this is known as a nobiliary particle
however those peers of lower status may receive a Territorial designation

and the burgher class could have a non-nobiliary

Toponymic surname